Change locking of nodeGetUniqueFromIndexLookup

o Now guaranteed to hold a read lock if a node is existing or a write lock if no node was found
o Use a fresh index reader to see concurrent changes
o This commit fixes some code analysis warnings
o Ensure InMemoryIndex does not apply updates idempotently when not used as unique index
o Make sure that we work with index descriptors correctly w.r.t. schema
o Take schema read lock
o Use forceGetRecord when using loading schema rules
o Do not verify if a constraint index is online from Cypher
   (they always are and if not due to failure that is detected by kernel)
o Add embedded example on how to use Cypher to create unique nodes to manual
Merge pull request #1470 from boggle/2.0-node-get-unique

Fix locking of nodeGetUnique